Antiserum against formalin killed whole cells of Streptococcus mutans was raised in rabbits. The antiserum was added to the drinking water of one group of weanling conventional Osborne Mendel rats. Their controls received drinking water containing non-immune rabbit serum. Five days after the initiation of passive immunization all animals were infected with the homologous S. mutans strain. After 8 weeks the experiment was terminated, the teeth scored for caries and the numbers of S. mutans determined. There was a significant reduction in the number of S. mutans in the immunized group however, no significant difference was detected in the level of caries. This probably resulted from the high level of non-S. mutans background caries activity seen in these animals.